Since the advent of the space age, which began after the World War II era, the use of liquid fuel rocket engines has become commonplace. These engines use cryogenic fuels, such as liquid oxygen and liquid hydrogen. Because of the very low temperatures of these fuels in the liquid state, insulated fuel tanks were needed. The application of spray-on foam insulation has proved to be the cheapest and best way to put the required insulation on the tanks. Sometimes, however, this foam insulation does not adhere well to the metal fuel tanks. Therefore, these debonded areas must be detected and repaired so that they will not cause problems connected with the firing and use of the high altitude rockets.
At the present time, NASA has a requirement for a test system to evaluate the bonding of the insulation on the external tank for the Space Shuttle. The required testing system should: (1) be able to detect debonded areas consistent with the current quality control criteria for the external tank, (2) be non-destructive in nature, (3) include or be compatible with a method of verifying debonded areas, (4) be of a nature which could easily be used on a space vehicle, (5) have an output signal which could easily be interpreted by a technician, (6) have an output signal that could be permanently recorded, and (7) be able to test a substantial number of points in a large area in a short time.
One patent which issued to the National Aeronautics and Space Administration some years ago was U.S. Pat. No. 3,521,982 to Clotfelter et al. This device uses a variable frequency oscillator and an electro-mechanical transducer to transmit mechanical vibrations into low density insulation attached to a high density panel and receive back reflections from the low density material. The frequency of the oscillator was varied until it coincided with one of the resonant frequencies of the low density material. These resonant frequency reflections were displayed on a screen, and the amplitude of the resulting waveform provided an indication of whether the insulation was properly bonded. This system was certainly a step in the right direction, but the system did not work well enough to provide completely satisfactory operational results.
Another prior art patent is U.S. Pat. No. 3,106,838 to Crooks, which discloses a system for testing a welded joint between two pieces of metal. The Crooks device continuously impacts one of the metal pieces with an electric hammer at a frequency of two to eight times per second. A probe having an attached crystal is used to detect vibrations in the other piece of metal and change these vibrations to an electric current. This current is fed directly to an oscilloscope and displayed either as (1) a smooth and gradually decaying waveform (which indicates a good weld), or (2) a pulsating and decaying waveform (which indicates a poor weld). In the former case, the workpiece vibrates at one frequency indicating it is one piece the size of the complete welded object. In the latter case, the two pieces tend to vibrate at different frequencies, the two signals having varying phase relationships so that they will tend to interfere with each other, amplifying in some instances and dampening the vibrations in others.
U.S. Pat. No. 3,653,373 to Batterman discloses an apparatus for acoustically determining periodontal health. Batterman teaches the impacting of a tooth with an impacting device and positioning a microphone on the opposite side of the tooth in order to pick up vibrations from the tooth in the form of sound waves. Specifically, it is sound waves having the resonant frequency of the tooth which are picked up. This resonant frequency then gives an indication of whether or not the tooth is solidly rooted in its socket.
U.S. Pat. No. 3,967,498 to Pezzillo discloses a tire defect detector. The Pezzillo device has a roller with an attached handle. Inside the roller is a sound generating device comprising a hammer which hits an anvil. In operation, the Pezzillo device is placed inside a tire casing and sound is generated. The echo or return resonant signal is picked up by a microphone, and the audio signal is changed into an electrical signal. The electrical signal is compared to a predetermined scale in order to provide an indication as to whether or not the tire casing is sound.
All the inventions disclosed in the above-mentioned prior art patents have at least some utility as non-destructive test devices. However, none of the prior art patents disclose a device which could accomplish the purpose which is required of the instant invention. These prior art devices all appear to use a resonant or "signature" frequency to determine the condition of the workpiece. Conversely, as will be disclosed below, the present invention does not use a resonant frequency, but instead uses only frequency data which is sensed while the impact to the workpiece is actually taking place. Moreover, the present invention does not use a microphone to sense resonant sound waves as does Batterman and Pezzillo.
Therefore, the object of this invention is to provide a non-destructive insulation bond test system which would operate on one side of the workpiece only to locate debonded areas and also check the quality of bonds in bonded or partially bonded areas. A further object is that the part of the system which tests the workpiece be portable. A still further object is that the system provide speed and a simple decision making process.